1. Field of the Invention
The present invention relates to a multi-beam scanning optical system and an image forming apparatus using the same and, more particularly, to a multi-beam scanning optical system preferably used for an image forming apparatus such as a laser beam printer and a digital copying machine, which can perform high-speed and high-quality printing with a relatively simple structure.
2. Related Art
Conventionally, a scanning optical system, which is used in an image forming apparatus such as a laser beam printer and a digital copying machine, guides light beams emitted from a light source to deflection means by incident optical means and images the light beams deflected by the deflection means in a spot shape on a photosensitive drum surface, which is a surface to be scanned, thereby optically scanning the photosensitive drum surface with the light beams.
In recent years, there is a growing demand for the image forming apparatus with higher speed as the image forming apparatus realizes higher performance and higher functionality. In order to meet such a demand, use of a plurality of light sources has been examined. For example, Japanese Patent Application Laid-open No. 9-54263 proposes a multi-beam scanning optical system having a multi-beam laser chip, which emits aligned plurality of laser beams from one chip, as a light source.
In such a multi-beam scanning optical system, synchronization detection optical means (BD optical system) is generally provided immediately before a position where an image signal is written in order to accurately control a writing position of an image.
FIG. 15 is a main part sectional view in a main scanning direction (main scanning sectional view) of a conventional multi-beam scanning optical system. In the figure, reference numeral 51 denotes a light source unit, which has, for example, two light-emitting portions (light sources) consisting of a semiconductor laser. The two light-emitting portions are spaced apart from each other in a main scanning direction and a sub scanning direction. Reference numeral 52 denotes an aperture stop, which forms light beams emitted from each light-emitting portion in a desired optimal beam shape. Reference numeral 53 denotes a collimator lens, which converts the light beams that have passed through the aperture stop 52 into substantially parallel light beams. Reference numeral 54 denotes a cylindrical lens, which has a predetermined refracting power only in the sub scanning direction. Note that each of the aperture stop 52, the collimator lens 53, the cylindrical lens 54 and the like constitutes an element of incident optical means 62.
Reference numeral 55 denotes deflection means, which consists of, for example, a rotary polygon mirror and is rotated in an arrow A direction in the figure at a constant speed by driving means (not shown) such as a motor. Reference numeral 56 denotes scanning optical means having an fθ characteristic, which has two lenses, first and second fθ lenses. The scanning optical means 56 establishes an optically conjugate relationship between the vicinity of a deflection surface 55a of a light deflector 55 and the vicinity of a photosensitive drum surface 57 as a surface to be scanned within a sub scanning cross section, thereby realizing a toppling correcting function.
Reference numeral 58 denotes a return mirror (hereinafter referred to as “BD mirror”), which reflects a plurality of synchronization signal detection light beams (“BD light beams”) for adjusting timing at a scanning start position on the photosensitive drum surface 57 toward a synchronization detection element 61 discussed below. Reference numeral 59 denotes a slit (hereinafter referred to as “BD slit”), which is disposed in a position equivalent to the photosensitive drum surface 57. Reference numeral 60 denotes a BD conjugate lens, which is for establishing a conjugate relationship between the BD mirror 58 and the synchronization detection element 61 and corrects surface toppling of the BD mirror 58. Reference numeral 61 denotes an optical sensor (hereinafter referred to as “BD sensor”) as the synchronization detection element. Note that each of the return mirror 58, the BD slit 59, the BD conjugate lens 60, the BD sensor 61 and the like constitutes an element of the synchronization detection optical means (BD optical system).
In the figure, BD detection is performed for each BD light beam, and the timing at a scanning start position of image recording on the photosensitive drum surface 57 is adjusted for each BD light beam using an output from the BD sensor 61.
Incidentally, in the multi-beam scanning optical system with a plurality of light-emitting portions (light sources), an image to be printed is deteriorated if a relative writing position deviates for each of the light-emitting portions due to various reasons.
For example, Japanese Patent Application Laid-open No. 2000-235154 describes that it is a problem that a width of each light beam corresponding to each light-emitting portion is changed by the deflection means and, as a result, a difference in outputs of the BD sensor occurs to deviate a writing position.
However, a relative writing position may deviate even in a state in which a difference in outputs of the BD sensor corresponding to each light-emitting portion does not occur as described above. For example, a relative writing position deviates even in a case where a focus of the main scanning cross section of the BD light beams is out of position on the BD slit surface.
It is an object of the present invention to provide a multi-beam scanning optical system and an image forming apparatus using the same that can realize high-speed and high-quality printing by a relatively simple structure.